Lucasfilm might not have really revealed what happened to Luke Skywalker's (Mark Hamill) green lightsaber in Disney's Star Wars sequel trilogy, but it seems like the answer lies in the novelization for Star Wars: The Last Jedi.
Star Wars: The Rise of Skywalker might have closed the curtain on Lucasfilm's sequel trilogy, however, there are still a number of questions left unanswered, and that includes the question of the whereabouts of Luke Skywalker's green lightsaber. The only time the lightsaber ever appeared in the sequels was in a flashback scene in The Last Jedi where we get to see Luke approaching a sleeping Ben Solo's (Adam Driver) tent after sensing darkness in his nephew. The Jedi Master activates the lightsaber out of instinct, and though he turns it off, the young Ben Solo wakes up believing that his uncle is about to kill him. Frightened and angered, Ben Solo destroys Luke's Jedi temple, and we never get to see the saber again.
While some fans believe that R2-D2 recovered the lightsaber and kept it in its storage compartment, the novelization for The Last Jedi suggests another possibility (via Screen Rant) – the weapon could be with the Caretakers on Ahch-To.
The Caretakers took many of Luke's possessions when he came to the planet. Among those mentioned in the book are the Jedi Master's star compass and his "weapon." Though "green lightsaber" isn't explicitly named, we can assume that it might be the missing Jedi weapon. What other weapons could Luke have with him coming to Ahch-To?
Of course, Disney could change this answer in new books and series. We'll just have to see what comes up.
Star Wars: The Rise of Skywalker is currently screening in cinemas.